M. Hohwy et al., Fivefold symmetric homonuclear dipolar recoupling in rotating solids: Application to double quantum spectroscopy, J CHEM PHYS, 110(16), 1999, pp. 7983-7992
We describe a new fivefold symmetric approach to homonuclear recoupling in
rotating solids that is based on rotor-synchronized, spin-lock rf irradiati
on of the type employed previously in MELODRAMA and C7 (and their derivativ
e sequences) for C-13-C-13 recoupling. The fivefold sequence, like its seve
nfold relatives, is gamma-encoded, and therefore exhibits a theoretical eff
iciency of 73% for double quantum filtering (2QF). However, since the ratio
of rf field strength, omega(rf)/2 pi, to spinning frequency, omega(r)/2 pi
, is lower, it is possible to operate the sequence at higher spinning rates
, and we have investigated the 2QF efficiency as a function of H-1 decoupli
ng field strength at high spinning frequencies. We observe dramatic oscilla
tions of the recoupled signal with a period similar to omega(r)/2 pi indica
ting that the H-1 reservoir is behaving partially inhomogeneously. This kin
d of double quantum recoupling is explored in multiple spin systems and we
derive analytical forms for polarization transfer and double quantum excita
tion relevant for uniformly labeled systems. Finally, the wide applicabilit
y of the fivefold sequence is demonstrated with INADEQUATE type spectra of
uniformly C-13 labeled sucrose and L-alanine. (C) 1999 American Institute o
f Physics. [S0021-9606(99)01615-3].